(1) Field of the Invention
The present invention relates to a copying machine which performs two-sided copying, more particularly to a digital copying machine having a circular-type duplex function.
(2) Related Art
Digital copying machines having a circular-type duplex function have been proposed in the past. With the digital copying machine of this type, two-sided copying in a two-sided copy mode is performed in the following manner.
A copying sheet fed from a sheet cassette is transported to a toner image transferring position, where a toner image is transferred onto one side of the copying sheet. The copying sheet is then transported to a fixing unit, by which the toner image is fixed onto the sheet surface with heat. The copying sheet with an image copied on one side is returned to the toner image transferring position through a sheet re-feeding path including a sheet turning over device. Another image is ther formed on the other side of the copying sheet in the same manner. The copying sheet with an image formed on each side is discharged outside the copying machine without passing through the sheet re-feeding path. The passage starting from and ending back in the toner image transferring position via the sheet re-feeding path is called a circular transport path. When performing two-sided copying on a plurality of copying sheet successively, copying sheets simultaneously containable in the circular transport path are fed from a sheet cassette, and two-sided copying is performed on all the sheets. The same number of copying sheets are then fed from the sheet cassette, and the same process is repeated until two-sided copying is completed on a predetermined number of copying sheets.
When a one-sided copy mode is selected, copying sheets with an image formed on one side are discharged outside the copying machine without passing through the sheet re-feeding path.
With the copying machine having the above structure, an interval between copying sheets is determined in advance so that the heat-fixing of a toner image is not adversely affected when performing one-sided copying on a plurality of copying sheets successively. In the case of large size sheets, the sheet intervals are made long so as to prevent the fixing unit from cooling, while in the case of small size sheets, the sheet intervals are made short.
With the conventional copying machine, however, there is a problem that the two-sided copying speed is sometimes lower than the one-sided copying speed, depending on the sheet size in successive copying.